The fabrication of microminiature devices and circuits often requires the precise alignment of a mask with respect to a semiconductor wafer. For very high resolution devices, submicron alignment tolerances are often necessary. Such alignment is of particular importance for the use of photolithographic step-and-repeat projection printers such as the GCA Mann DSW 4800.
For alignment, it is customary to employ alignment marks on the wafer surface.
One type of preferred alignment mark is the Fresnel zone plate. (See Klein, M. V., Optics, John Wiley and Sons, Inc., New York, 1970, for a general discussion of Fresnel zones and Fresnel zone plates. See also Feldman, M. et al, U.S. Pat. No. 4,037,969, for a discussion of the use of Fresnel zone plates as alignment marks on a semiconductor wafer.)
Illustratively, a Fresnel zone plate alignment mark located on the surface of a semiconductor wafer comprises alternating concentric annular regions of low and high reflectivity. Alternatively, the zone plate alignment mark comprises alternating concentric annular regions which cause reflected light to undergo destructive and constructive interference. Such patterns may be formed in a variety of ways known to the art such as by direct electron beam exposure or by exposing photoresist to light. Wafers to be aligned for use in a step-and-repeat system usually include two or more alignment marks on a surface portion of the wafer. For chip-by-chip alignment there is an alignment mark on each chip.
The Fresnel zone plate alignment marks have a unique optical property which enables them to act simultaneously as a positive lens with focal lengths of f, f/3, f/5 . . . and as a negative lens with focal lengths of -f, -f/3, -f/5 . . . The exact value of f is determined by the geometry of the plates. (See Feldman, supra.) Thus, incident radiation collimated parallel to the optical axis is focused by a Fresnel zone plate alignment mark into a plurality of focused real images at distances, f, f/3, f/5 . . . in front of the wafer surface and a plurality of focused virtual images located at distances -f, -f/3, -f/5 . . . behind the wafer surface. In this specification, distances in front of the wafer surface are taken as positive and distances behind the wafer surface are taken as negative.
Illustratively, to align the semiconductor wafer in accordance with a typical prior art technique, radiation is directed onto a Fresnel zone plate alignment mark to form a plurality of focused real images located at prescribed distances in front of the wafer surface and a plurality of focused virtual images located at prescribed distances behind the wafer surface. A selected one of the focused images, usually the real image associated with the +f focal length or the virtual image associated with the -f focal length, is projected by an optical system onto a four-quadrant photodetector arrangement which, along with appropriate electronic circuitry, serves as a position sensing means. The position sensing means is adapted to determine when the projected image is substantially coincident with a preselected location which illustratively is at the origin of the four-quadrant arrangement. In response to output from the position sensing means, the wafer is then moved so that the projected image coincides with the preselected location, thus achieving alignment.
The accuracy of the above-mentioned prior art zone plate alignment technique is limited by local wafer tilt. Local wafer tilt generally refers to deviations from planar geometry on the wafer surface instead of bulk rotation of the wafer. Local wafer tilt displaces both the real and virtual images formed by the zone plates, thereby producing systematic error. For example, if the zone plate alignment mark is formed on a portion of the wafer surface having a local tilt, the real image associated with the +f focal length can be displaced so that its projection on a photodetector arrangement coincides with a preselected location, even though a similar alignment mark on a tilt-free planar surface would not produce an image, associated with the +f focal length, whose projection on the photodetector arrangement coincides with the preselected location, thereby introducing a systematic error. Illustratively, a local wafer tilt of about 1 .mu.m/cm can lead to alignment errors on the order of 0.06.mu. for 300.mu. focal length zone plates.
A similar problem arises if photoresist covering the semiconductor wafer is of nonuniform thickness. In this case, the refraction of light in regions of nonuniform photoresist thickness can lead to the displacement of images formed by alignment marks on the wafer surface, thereby resulting in systematic alignment errors.
In view of the above, efforts have been directed to finding a way to compensate for local wafer tilt and/or nonuniform photoresist thickness in the alignment of semiconductor wafers for patterning by step-and-repeat photolithographic systems.